If X is the brother of the son of Y’s son, how is X related to Y?

Concept: In family-relation puzzles, each “son of” (or “daughter of”) link moves one generation down from the person named. Two people described as each other’s brother/sister share the same parent, so they occupy the same generation and hold the identical relation to any ancestor above that shared parent.

Application:

1. Let Y’s son be A — A is one generation below Y.

2. “The son of Y’s son” is A’s son — call him B. B is one generation below A, so B is two generations below Y, i.e. Y’s grandson.

3. X is given as the brother of B, so X shares the same parent (A) as B and occupies the same generation as B.

4. Since B is Y’s grandson and X is B’s sibling, X is also Y’s grandson.

Cross-check: Counting the links directly from X to Y gives X → parent A → grandparent Y, i.e. exactly two “son of” steps, which is the definition of a grandson — matching the result above.

Answer: Grandson.